Method for cross-linking polycrylates

ABSTRACT

A method for producing cross-linked polyacrylates, wherein a polymer is produced by copolymerizing an acrylate-based monomer mixture and the polymer is cross-linked by means of UV radiation, wherein up to 10 wt % of the monomers have one or more epoxy and/or hydroxyl groups which are incorporated into the polymer during copolymerization, and wherin at least one photocaton generator and one or more di-or multifunctional epoxides and/or alcohols are added to the polymers before cross-linking.

[0001] The invention relates to the crosslinking of pressure sensitivelyadhesive polyacrylate hotmelts with di- or polyfunctional epoxidesand/or with di- or polyfunctional alcohols.

[0002] In the field of pressure sensitive adhesive (PSA) compositions,ongoing technological developments in the coating technique mean thatthere is a progressive need for new developments. Within the industry,hotmelt processes with solventless coating technology are of increasingimportance in the preparation of PSA compositions, since theenvironmental regulations are becoming ever greater and the prices ofsolvents continue to rise. The consequence of this is that solvents areto be eliminated as far as possible from the manufacturing process forPSA tapes. The associated introduction of the hotmelt technology isimposing ever greater requirements on the adhesive compositions. AcrylicPSA compositions in particular are the subject of very intensiveinvestigations aimed at improvements. For high-level industrialapplications, polyacrylates are preferred on account of theirtransparency and weathering stability. In addition to these advantages,however, these acrylic PSA compositions may also meet stringentrequirements in respect of shear strength and bond strength. Thisprofile of requirements is met by polyacrylates of high molecular weightand high polarity with subsequent efficient crosslinking. For thecrosslinking there are in principle two methods available, since thethermal crosslinking of acrylic hotmelt PSAs can be realized only bycircuitous routes. Electron beam crosslinking offers the advantage thatcrosslinking can still be carried out at relatively high adhesiveapplication rates. The disadvantage is the inefficient crosslinking,since there are no reactive groups, such as double bonds, for example.Consequently, the quality of electron beam crosslinking of polyacrylatesis always relatively poor.

[0003] UV crosslinking requires less elaborate apparatus and istherefore of advantage for industrial application. In this case,free-radical intermediates are formed which react with one another andlead to crosslinking of the PSAs.

[0004] U.S. Pat. No. 4,717,605 describes a method of adhesively bondingoptical glass components. It does so using radiation-curable adhesivesbased on ionically polymerizable epoxy systems and ionic photoinitiatorsbased on triarylsulfonium complex salts. These adhesives still containat least one ethylenically unsaturated compound, which can bepolymerized by free radicals in the presence of a free-radicalphotoinitiator.

[0005] WO 88/02879 uses a photoinitiator and an iron salt for cationicphotopolymerization. That document discloses a polymerizable compositioncomposed of a free-radically polymerizable material and a photocatalyst.The photoinitiator system is composed of π-aromatic-metal complexes ofthe form

(R⁶—Fe⁺—R⁷)LZ_(k) ⁻,

[0006] where R⁶ is an η⁶-aromatic, R is the anion of a cyclopentadienylcompound, L is a di- to heptavalent metal or metalloid, Z is a halogen,and k is the valence of L increased by 1. The photocatalyst systemfurther comprises a peroxidic or hydroperoxidic compound and,optionally, a metallocene.

[0007] U.S. Pat. No. 5,776,290 claims a method of preparing a coatedabrasive article, where a first binder is present on a backing and amultiplicity of abrasive particles are present in this binder. Thebinder is composed of a pressure sensitive hotmelt adhesive film. Thishotmelt adhesive is cured by means of an energy source so that theabrasive particles are covered by a crosslinked coat of the adhesive.The hotmelt adhesive crosslinked with an epoxide in this document isbased on pressure sensitively adhesive polyesters.

[0008] The UV-initiated epoxide crosslinking reaction, as a highlyefficient crosslinking reaction, has not been successfully transferredto date to polyacrylate PSAs.

[0009] It is an object of the invention to provide a process forcrosslinking polyacrylates, especially polyacrylate-based hotmeltadhesives, which does not have the disadvantages of the prior art andwhich advantageously extends the state of the art.

[0010] This object is achieved by a process for crosslinkingpolyacrylates in accordance with the main claim. It has been found,surprisingly and completely unexpectedly for the skilled worker, thatpolyacrylates can be crosslinked outstandingly by UV-initiated epoxidecrosslinking and in terms of shear strength have advantages overconventional crosslinking mechanisms, especially electron beamcrosslinking, if they have been functionalized with appropriate groupsduring polymerization. The subclaims relate to advantageous developmentsof the invention. A second independent claim relates to the use ofdifunctional or polyfunctional oxygen compounds, especially difunctionalor polyfunctional epoxides or alcohols, as crosslinking reagent forfunctionalized polyacrylates, especially functionalized acrylic hotmeltpressure sensitive adhesives.

[0011] The main claim accordingly provides a process for preparingcrosslinked polyacrylates, in which an acrylate-based monomer mixture iscopolymerized to produce a polymer and, after the polymerization,crosslinking of the polymer is brought about by UV radiation, with up to10% by weight of copolymerizable monomers containing one or more epoxygroups and/or one or more hydroxyl groups being incorporated into thepolymers during the copolymerization and, prior to crosslinking, atleast one photocation generator and also one or more di- orpolyfunctional epoxides and/or one or more di- or polyfunctionalalcohols are added to the polymers.

[0012] Preferably, the free di- or polyfunctional epoxides and/or thefree di- or polyfunctional alcohols are used such that one of thesecomponents is present in a marked excess over the other (by freeepoxides or alcohols are meant here the functionalized monomers notincorporated into the polymer chain). In the case of an excess of freeepoxides it is advantageous if not more than 10 mol % of hydroxyl groupsare present in the form of free alcohols, based on the epoxy groups ofthe free epoxides; in the case of an excess of free alcohol it isfavorable to choose the fraction of the epoxy groups in the form of freeepoxides to be not above a fraction of 10 mol %, based on the hydroxylgroups of the free alcohol.

[0013] Where di- or polyfunctional free alcohols are present it is verypreferable not to add any di- or polyfunctional free epoxides;correspondingly, where di- or polyfunctional free epoxides are present,it is very advantageous to avoid the presence of di- or polyfunctionalfree alcohols.

[0014] In one first advantageous embodiment of the inventive process thecomposition of the monomer mixture is such that the resulting polymerspossess pressure sensitively adhering properties.

[0015] The polyacrylates to be crosslinked are preferably polyacrylateswhich have been processed and worked on prior to crosslinking in thehotmelt process.

[0016] Furthermore, the inventive process is very favorable if thecomposition of the monomer mixture is as follows:

[0017] a) 60 to 99.5% by weight of (meth)acrylic acid and/or(meth)acrylate esters of the following formula

[0018]  where R¹=H or CH₃ and R² is an alkyl chain having 1 to 20 carbonatoms

[0019] b) 0.5 to 10% by weight of copolymerizable monomers which containone or more epoxy groups and/or one or more hydroxyl groups,

[0020] c) optionally up to a maximum of 39.5% by weight ofcopolymerizable olefinically unsaturated monomers containing functionalgroups which activate the double bond for the polymerization reaction,

[0021] the components of the monomer mixture adding up to 100%,

[0022] and/or if, prior to crosslinking,

[0023] d) 0.01 to 25% by weight of a photocation generator and

[0024] e) 0.1 to 5% by weight of one or more di- or polyfunctionalepoxides and/or one or more di- or polyfunctional alcohols

[0025] are added to the polymers, so that the polymers and components d)and e) add up to 100% by weight.

[0026] The concept of the invention is to copolymerize monomers havingeither epoxy groups and/or hydroxyl groups into the polyacrylates duringthe polymerization. In the presence of suitable crosslinkers, especiallythe above-described di- or polyfunctional epoxides and/or di- orpolyfunctional alcohols for polyacrylates modified with epoxy groupsand/or with hydroxyl groups, it is then possible by way of thesefunctional groups to achieve crosslinking with exposure to ultravioletradiation.

[0027] In this way it is possible to use such crosslinkers for thecationic curing or crosslinking of the functionalized polyacrylates.

[0028] In one advantageous development of the inventive process, incomponent a) of the monomer mixture, the radical R² represents an alkylchain having 4 to 14 carbon atoms, preferably having 4 to 9 carbonatoms.

[0029] Specific examples of such acrylic monomers, which can be usedvery advantageously, are n-butyl acrylate, n-pentyl acrylate, n-hexylacrylate, n-heptyl acrylate, n-octyl acrylate, n-nonyl acrylate, and thebranched isomers thereof, such as 2-ethylhexyl acrylate, for example.

[0030] Further vinyl monomers, which can be used advantageously incombination with acrylate monomers in the sense of the component c),include vinyl esters, vinyl ethers, vinyl halides, vinylidene halides,and nitriles of ethylenically unsaturated hydrocarbons. Specificexamples are vinyl acetate, N-vinylformamide, vinylpyridine,acrylamides, ethyl vinyl ether, vinyl chloride, vinylidene chloride,acrylonitrile, maleic anhydride, and styrene.

[0031] The monomers are preferably chosen so that the polymer preparedhas adhesive properties in accordance with the Handbook ofPressure-Sensitive Adhesives, p. 172, § 1, 1989.

[0032] For the crosslinking reaction presented, the photocationinitiators familiar to the skilled worker are used, preferably one ofthe initiators from the group presented below.

[0033] As photocation generators (“photoinitiators”) it is preferred toemploy aryldiazonium salts (“onium salts”) which can be representedgenerally by the formula Ar-N=N⁺LX⁻, LX⁻ being an adduct of a Lewis acidL and a Lewis base X⁻. Particularly advantageous for LX⁻ are BF₄ ⁻, SbF₅⁻, AsF₅ ⁻, PF₅ ⁻, SO₃CF₂ ⁻. Under the influence of UV radiation there isa rapid cleavage of the molecule into the aryl halide (ArX), nitrogen,and the corresponding Lewis acid.

[0034] Additionally, aryliodonium salts (C₆H₅)RI⁺LX⁻, where R is anorganic radical, especially diaryliodonium salts (C₆H₅)₂I⁺LX⁻, and alsotriarylsulfonium salts (C₆H₅)₃S⁺ LX⁻, are known for use as cationicphotoinitiators. In the presence of proton donors these salts formstrong (Brönstedt) acids, which are likewise highly suitable for theinitiation of cationic polymerizations and for the inventive process.

[0035] Sulfonium salts as cation photoinitiators are present, forexample, also in the form of the compounds H₅C₆—CO—CH₂—S⁺ LX⁻ orH₅C₆—CO—CH₂-Pyr⁺ LX⁻, with Pyr representing a nitrogen-containingheteroaromatic system (e.g., pyridine, pyrimidine).

[0036] In one very advantageous embodiment of the inventive process thephotocation generator chosen is a triarylsulfonium hexafluoro salt fromgroup 15 of the periodic system, preferably such that the element fromgroup 15 is present in the IV oxidation state. Use is made veryfavorably of triarylsulfonium hexafluorophosphate and/ortriarylsulfonium hexafluoroantimonate.

[0037] In one very advantageous development of the inventive process astructuring of the crosslinked polyacrylates is achieved in the courseof crosslinking by covering the polyacrylates to be crosslinked with amask having regions of different UV transparency. In this case,irradiation is carried out with ultraviolet light such that certainregions of the polymer mixture are subject to different intensities ofradiation. The structuring of the polyacrylates consists in regions ofhigh crosslinking being present alongside regions of low crosslinkingand/or noncrosslinked regions within the polyacrylates.

[0038] Also claimed is the use of di- or polyfunctional oxygencompounds, especially di- or polyfunctional epoxides or alcohols, ascrosslinking reagents for the crosslinking reaction, brought about byultraviolet radiation in the presence of a photocation generator, ofpolyacrylates functionalized by epoxy groups and/or by hydroxyl groups.

[0039] The basic principles of the invention are set out below. Thepolymers for crosslinking are prepared from the monomer mixture byfree-radical polymerization such that their molecular weight lies withinthe order of magnitude of 250,000-1,000,000 g/mol.

[0040] The free-radical polymerization may be conducted in the presenceof one or more organic solvents and/or in the presence of water orwithout any solvent. It is preferred to use as little solvent aspossible. Depending on conversion rate and temperature, thepolymerization time is between 6 and 48 h.

[0041] In the case of solution polymerization, solvents used arepreferably esters of saturated carboxylic acids (such as ethyl acetate),aliphatic hydrocarbons (such as n-hexane or n-heptane), ketones (such asacetone or methyl ethyl ketone), special boiling point spirit, ormixtures of these solvents. As polymerization initiators use is made ofcustomary free-radical-forming compounds, such as peroxides and azocompounds, for example. Initiator mixtures as well can be used. In thepolymerization it is further possible to use thiols as regulators forlowering the molecular weight and reducing the polydispersity. As whatare known as polymerization regulators it is possible to use, forexample, alcohols and ethers.

[0042] In one very favorable procedure, after the polymerization, thePSA is coated from solution onto a backing material. In one verypreferred variant the polymerization medium is removed under reducedpressure, this operation being carried out at elevated temperatures, inthe range from 80 to 150° C., for example. The polymers can then be usedand coated in the solvent-free state, in particular as hotmelt PSAs. Forselected applications it is also of advantage to prepare and process thepolymers without any solvent.

[0043] The blends described in this invention can be modified further inorder to achieve the optimum technical adhesive properties.

[0044] By way of example, the polymers for preparing PSAs are optionallyblended with one or more resins. Examples of resins which can be usedinclude terpene resins, terpene-phenolic resins, C₅ and C₉ hydrocarbonresins, pinene resins, indene resins, and rosins, alone and also incombination with one another, and also their disproportionated,hydrogenated, polymerized, and esterified derivatives and salts. Inprinciple, however, it is possible to use any resins which are solublein the corresponding polyacrylate; reference may be made in particularto all aliphatic, aromatic, and alkylaromatic hydrocarbon resins,hydrocarbon resins based on pure monomers, hydrogenated hydrocarbonresins, functional hydrocarbon resins, and natural resins.

[0045] Furthermore, it is possible to add various fillers (for example,carbon black, TiO₂, solid or hollow beads of glass or other materials,nucleators), blowing agents, compounding agents and/or aging inhibitors.

[0046] In one particular advantageous development, plasticizers areadmixed in order to improve the flow behavior of the PSA.

[0047] A development which makes the process of the inventionparticularly advantageous for the preparation of, for example, adhesivetapes is distinguished by the further processing of the PSA from themelt.

[0048] As backing material, for adhesive tapes for example, it ispossible to use the materials which are customary and familiar to theskilled worker, such as films (polyester, PET, PE, PP, BOPP, PVC),nonwovens, foams, wovens and woven films, and also release paper(glassine, HDPE, LDPE).

[0049] The inventive crosslinking of the polyacrylates or of the(hotmelt) PSAs takes place by brief UV irradiation in the range from 200to 400 nm using commercially customary high-pressure or medium-pressuremercury lamps with an output of, for example, 80 to 200 W/cm. It may beappropriate to adapt the lamp output to the belt speed or to run thebelt slowly while shading it off partly in order to reduce the thermalload thereon. The irradiation time is guided by the construction andoutput of the respective lamps.

[0050] The inventive process can be utilized outstandingly to preparestructure polyacrylates, especially structured (hotmelt) PSAs. Oneprocess for preparing structured polyacrylates by structuredcrosslinking of polyacrylate mixtures is distinguished in that the basepolymer mixture is irradiated with ultraviolet light in such a way thatonly certain regions of the polymer mixture are exposed to the UVradiation.

[0051] The process for preparing structured polyacrylates may beconducted in particular such that the base polymer mixture is irradiatedwith ultraviolet light through a perforated mask in such a way that onlycertain regions of the polymer mixture are exposed to the UV radiation.

[0052] Alternatively, the structuring of the polymer mixture for curingmay be achieved by using, rather than the perforated mask, a film whosetwo-dimensional extent has regions of different UV transparency, so thatcertain regions of the polymer mixture are exposed to differentintensities of the UV radiation.

[0053] In FIG. 1, the principle of selective irradiation is illustratedby a diagram. In the figure, the irradiation of the acrylate composition(2) through a perforated mask (1) is shown, the acrylate composition (2)being present on the backing (3). In accordance with the main claim, theacrylate composition (1) is admixed with a photocation initiator whichinitiates the crosslinking reaction as a result of UV light (4). Theultraviolet rays (4) are able to penetrate the mask (1) only in theregion of the perforations (11), so that after irradiation the situationdepicted in the bottom part of the diagram results: the pressuresensitive adhesive (2) has hard segments of high crosslinking (21)alongside noncrosslinked segments (22).

[0054] The polymer chains at the margins of the hard regions extend intothe soft regions; accordingly, the hard regions, which are inherently ofhigh viscosity, are linked with the soft regions and so hinder theseregions in their mobility, so that the structural strength of theadhesive is increased. Moreover, these hard segments increase thecohesion of the pressure sensitive adhesive. In contrast, the softsegments facilitate the flow of the adhesive on the substrate and soincrease the bond strength and the tack. A great influence on thetechnical adhesive properties is exerted by the percentage fraction ofthe irradiated surface area, and also by the size of the segmentsgenerated.

EXAMPLES

[0055] The following exemplary experiments are intended to illustratethe content of the invention, without wishing to restrict the inventionunnecessarily through the choice of the examples.

[0056] Test Methods

[0057] The polyacrylate compositions and their crosslinked products werecharacterized using the test methods described below:

[0058] Shear strength (test A1, A2)

[0059] A 13 mm wide strip of the adhesive tape was applied to a smoothand cleaned steel surface. The application area was 20 mm×13 mm(length×width). The subsequent procedure was as follows:

[0060] Test Al: At room temperature, a 1 kg weight was fastened to theadhesive tape and the time which elapsed until the weight fell off wasmeasured.

[0061] Test A2: At 70° C., a 1 kg weight was fastened to the adhesivetape and the time which elapsed until the weight fell off was measured.

[0062] The measured shear stability times are each reported in minutes,and correspond to the average of three measurements.

[0063] 180° bond strength test (test B)

[0064] A 20 mm wide strip of an acrylic PSA coated onto a polyester wasapplied to steel plates. The PSA strip was pressed onto the substratetwice using a 2 kg weight. The adhesive tape was then peeled immediatelyfrom the substrate at 300 mm/min and at an angle of 180°. The steelplates were washed twice with acetone and once with isopropanol. For themeasurements on the PE substrate, only new plates were used. The resultsare reported in N/cm and are averaged from three measurements. Allmeasurements were made at room temperature under climatized conditions.

[0065] Determination of the gel fraction (test C)

[0066] The carefully dried, solvent-free adhesive samples are weldedinto a nonwoven polyethylene (Tyvek) pouch. The gel value (weightfraction of the polymer that is not soluble in toluene) is determinedfrom the difference in the sample weights before and after extractionwith toluene.

[0067] Samples Investigated

[0068] The samples used for the experiments were prepared as follows:

[0069] The polymers were prepared conventionally by free-radicalpolymerization; the average molecular weight is approximately 800,000g/mol.

Example 1

[0070] A 2 L glass reactor conventional for free-radical polymerizationswas charged with 8 g of glycidyl methacrylate, 196 g of n-butylacrylate, 196 g of 2-ethylhexyl acrylate and 266 g ofacetone/isopropanol (97:3). After nitrogen gas had been passed throughthe reaction solution with stirring for 45 minutes, the reactor washeated to 58° C. and 0.4 g of AIBN [2,2′-azobis(2-methylbutyronitrile)]was added. Subsequently, the external heating bath was heated to 75° C.and the reaction was conducted constantly at this external temperature.After 4 and 6 h, dilution was carried out with in each case 150 g ofacetone/isopropanol mixture (97:3). After a reaction time of 48 h, thepolymerization was terminated and the reaction vessel was cooled to roomtemperature.

[0071] The product was subsequently diluted with 150 g of acetone, and12.8 g of bisphenol A and 8 g of triarylsulfonium hexafluoroantimonate(50% strength solution in propylene carbonate; Cyracure UVI-6994® [UNIONCARBIDE]) were added. The mixture was coated from solution at 50 g/m²onto an isocyanate-primed PET film and heated at 120° C. for 10 minutes.UV irradiation was carried out using a Xenon chloride lamp (VIB 308 bulb[FUSION]) with a radiation intensity of 160 W/m². After one lamp passwith a belt speed of 20 m/min, the adhesive tape specimens were furtherheated for 10 minutes and then tested in accordance with test methods Ato C.

Example 2

[0072] The procedure of Example 1 was repeated. The polymer wassubsequently diluted with 150 g of acetone, and 12.8 g of bisphenol Aand 8 g of triarylsulfonium hexafluorophosphate (50% strength solutionin propylene carbonate; Cyracure UVI-6990® [UNION CARBIDE]) were added.The mixture was coated from solution at 50 g/m² onto anisocyanate-primed PET film and heated at 120° C. for 10 minutes. UVirradiation was carried out using a xenon chloride lamp (VIB 308 bulb[FUSION]) with a radiation intensity of 160 W/m². After one lamp passwith a belt speed of 20 m/min, the adhesive tape specimens were furtherheated for 10 minutes and then tested in accordance with test methods Ato C.

Example 3

[0073] The procedure of Example 1 was repeated. The polymer was coatedat 50 g/m² onto an isocyanate-primed PET film, dried at 120° C. for 10minutes, cured with electron beams (230 kV acceleration voltage, EBCunit from Crosslinking), and then subjected to technical adhesivetesting using test methods A to C.

Example 4

[0074] The procedure of Example 1 was repeated. 4 g of triarylsulfoniumhexafluoroantimonate (50% strength solution in propylene carbonate;Cyracure UVI-6994® [UNION CARBIDE]) were added to the blend.

Example 5

[0075] A 2 L glass reactor conventional for free-radical polymerizationswas charged with 8 g of glycidyl methacrylate, 4 g of acrylic acid, 194g of n-butyl acrylate, 194 g of 2-ethylhexyl acrylate and 266 g ofacetone/isopropanol (97:3). After nitrogen gas had been passed throughthe reaction solution with stirring for 45 minutes, the reactor washeated to 58° C. and 0.4 g of AIBN [2,2′-azobis(2-methylbutyronitrile)]was added. Subsequently, the external heating bath was heated to 75° C.and the reaction was conducted constantly at this external temperature.After 4 and 6 h, dilution was carried out with in each case 150 g ofacetone/isopropanol mixture (97:3). After a reaction time of 48 h, thepolymerization was terminated and the reaction vessel was cooled to roomtemperature.

[0076] The product was subsequently diluted with 150 g of acetone, and12.8 g of bisphenol A and 8 g of triarylsulfonium hexafluoroantimonate(50% strength solution in propylene carbonate; Cyracure UVI-6994®([UNION CARBIDE]) were added. The mixture was coated from solution at 50g/m² onto an isocyanate-primed PET film and heated at 120° C. for 10minutes. UV irradiation was carried out using a xenon chloride lamp (VIB308 bulb [FUSION]) with a radiation intensity of 160 W/m². After onelamp pass with a belt speed of 20 m/min, the adhesive tape specimenswere further heated for 10 minutes and then tested in accordance withtest methods A to C.

Example 6

[0077] A 2 L glass reactor conventional for free-radical polymerizationswas charged with 8 g of glycidyl methacrylate, 4 g of acrylic acid, 20 gof methyl acrylate, 20 g of N-tertbutylacrylamide, 348 g of 2-ethylhexylacrylate and 266 g of acetone/isopropanol (97:5). After nitrogen gas hadbeen passed through the reaction solution with stirring for 45 minutes,the reactor was heated to 58° C. and 0.4 g of AIBN[2,2′-azobis(2-methylbutyronitrile)] was added. Subsequently, theexternal heating bath was heated to 75° C. and the reaction wasconducted constantly at this external temperature. After 4 and 6 h,dilution was carried out with in each case 150 g of acetone/isopropanolmixture (97:5). After a reaction time of 48 h, the polymerization wasterminated and the reaction vessel was cooled to room temperature. Theaverage molecular weight is approximately 710,000 g/mol.

[0078] The product was subsequently diluted with 150 g of acetone, and12.8 g of bisphenol A and 8 g of triarylsulfonium hexafluoroantimonate(50% strength solution in propylene carbonate; Cyracure UVI-6994 (D[UNION CARBIDE]) were added. The mixture was coated from solution at 50g/m² onto an isocyanate-primed PET film and heated at 120° C. for 10minutes. UV irradiation was carried out using a xenon chloride lamp (VIB308 bulb [FUSION]) with a radiation intensity of 160 W/m². After onelamp pass with a belt speed of 20 m/min, the adhesive tape specimenswere further heated for 10 minutes and then tested in accordance withtest methods A to C.

Example 7

[0079] A 2 L glass reactor conventional for free-radical polymerizationswas charged with 8 g of hydroxyethyl methacrylate, 196 g of n-butylacrylate, 196 g of 2-ethylhexyl acrylate and 266 g ofacetone/isopropanol (97:3). After nitrogen gas had been passed throughthe reaction solution with stirring for 45 minutes, the reactor washeated to 58° C. and 0.4 g of AIBN [2,2′-azobis(2-methylbutyronitrile)]was added. Subsequently, the external heating bath was heated to 75° C.and the reaction was conducted constantly at this external temperature.After 4 and 6 h, dilution was carried out with in each case 150 g ofacetone/isopropanol mixture (97:3). After a reaction time of 48 h, thepolymerization was terminated and the reaction vessel was cooled to roomtemperature.

[0080] The product was subsequently diluted with 150 g of acetone, and20 g of bisepoxidized bisphenol A Rütapox 164™ (from Bakelite AG) and 8g of triarylsulfonium hexafluoroantimonate (50% strength solution inpropylene carbonate; Cyracure UVI-6994® ([UNION CARBIDE]) were added.The mixture was coated from solution at 50 g/m² onto anisocyanate-primed PET film and heated at 120° C. for 10 minutes. UVirradiation was carried out using a xenon chloride lamp (VIB 308 bulb[FUSION]) with a radiation intensity of 160 W/m . After one lamp passwith a belt speed of 20 m/min, the adhesive tape specimens were furtherheated for 10 minutes and then tested in accordance with test methods Ato C.

Example 8

[0081] The procedure of Example 7 was repeated. The polymer was coatedat 50 g/m² onto an isocyanate-primed PET film, dried at 120° C. for 10minutes, cured with electron beams (230 kV acceleration voltage, EBCunit from Crosslinking), and then subjected to technical adhesivetesting using test methods A-C.

Example 9

[0082] A 2 L glass reactor conventional for free-radical polymerizationswas charged with 8 g of hydroxyethyl methacrylate, 4 g of acrylic acid,194 g of n-butyl acrylate, 194 g of 2-ethylhexyl acrylate and 266 g ofacetone/isopropanol (97:3). After nitrogen gas had been passed throughthe reaction solution with stirring for 45 minutes, the reactor washeated to 58° C. and 0.4 g of AIBN [2,2′-azobis(2-methylbutyronitrile)]was added. Subsequently, the external heating bath was heated to 75° C.and the reaction was conducted constantly at this external temperature.After 4 and 6 h, dilution was carried out with in each case 150 g ofacetone/isopropanol mixture (97:3). After a reaction time of 48 h, thepolymerization was terminated and the reaction vessel was cooled to roomtemperature.

[0083] The product was subsequently diluted with 150 g of acetone, and20 g of bisepoxidized bisphenol A Rütapox 164™ (from Bakelite AG) and 8g of triarylsulfonium hexafluoroantimonate (50% strength solution inpropylene carbonate; Cyracure UVI-6994® [UNION CARBIDE]) were added. Themixture was coated from solution at 50 g/m² onto an isocyanate-primedPET film and heated at 120° C. for 10 minutes. UV irradiation wascarried out using a xenon chloride lamp (VIB 308 bulb [FUSION]) with aradiation intensity of 160 W/m². After one lamp pass with a belt speedof 20 m/min, the adhesive tape specimens were further heated for 10minutes and then tested in accordance with test methods A to C.

Example 10

[0084] A 2 L glass reactor conventional for free-radical polymerizationswas charged with 8 g of hydroxyethyl methacrylate, 4 g of acrylic acid,20 g of methyl acrylate, 20 g of N-tert-butylacrylamide, 348 g of2-ethylhexyl acrylate and 266 g of acetone/isopropanol (97:5). Afternitrogen gas had been passed through the reaction solution with stirringfor 45 minutes, the reactor was heated to 58° C. and 0.4 g of AIBN[2,2′-azobis(2-methylbutyronitrile)] was added. Subsequently, theexternal heating bath was heated to 75° C. and the reaction wasconducted constantly at this external temperature. After 4 and 6 h,dilution was carried out with in each case 150 g of acetone/isopropanolmixture (97:5). After a reaction time of 48 h, the polymerization wasterminated and the reaction vessel was cooled to room temperature. Theaverage molecular weight is approximately 686,000 g/mol.

[0085] The product was subsequently diluted with 150 g of acetone, and20 g of bisepoxide bisphenol A Rütapox 164™ (from Bakelite AG) and 8 gof triarylsulfonium hexafluorophosphate (50% strength solution inpropylene carbonate; Cyracure UVI-6990® [UNION CARBIDE]) were added. Themixture was freed from the solvent under reduced pressure and thencoated at 50 g/m² from the melt at 120° C. through a slot die onto anisocyanate-primed PET film. UV irradiation was carried out using a Xenonchloride lamp (VIB 308 bulb [FUSION]) with a radiation intensity of 160W/m². After one lamp pass with a belt speed of 20 m/min, the adhesivetape specimens were further heated for 10 minutes and then tested inaccordance with test methods A to C.

[0086] Results

[0087] The results of the technical adhesive tests of Examples 1 to 3are shown in Table 1. TABLE 1 SST 10 N, SST 10 N, 70° C. BS steel Gelvalue [min] [min] [N/cm] [%] Example (test A1) (test A2) (test B) (testC) 1 +10000 7885 3.8 70 2 +10000 +10000 3.7 65  3^(a) 255 30 4.0 43 3^(b) 1085 125 4.0 63

[0088] The investigations of Examples 1 and 2 demonstrate that with theprocess of the invention it is possible to prepare pressure sensitiveadhesives having a very high shear strength. Both triarylsulfoniumhexafluorophosphate and triarylsulfonium hexafluoroantimonate aresuitable photoinitiators for epoxy crosslinking. The epoxy functions areincorporated by polymerization in the polymer chain by means of glycidylmethacrylate. For crosslinking, bisphenol A is used. Example 3demonstrates the superiority of the crosslinking of the invention overthe conventional electron beam curing. With approximately the same gelvalue, the shear strength of the electron-beam-crosslinked samples liesbelow that of Examples 1 and 2.

[0089] The results of the technical adhesive evaluations of Examples 4to 6 are shown in Table 2. TABLE 2 SST 10 N, SST 10 N, 70° C. BS steelGel value [min] [min] [N/cm] [%] Example (test A1) (test A2) (test B)(test C) 4 +10000 +10000 3.6 69 5 +10000 +10000 3.6 73 6 +10000 +100003.5 70

[0090] Examples 4 to 6 underline the universal usefulness of thecrosslinking process of the invention for acrylic PSA. Thus, forexample, it is even possible to use acrylic acid as a comonomer and tocarry out epoxy crosslinking. Example 4 is evidence that even thephotoinitiator fraction can be reduced further for efficientcrosslinking. The shear strength is fully retained. It is also possibleto lower the average molecular weight. Example 6 possesses an averagemolecular weight of approximately 700,000 g/mol and yet achieves theoptimum shear strength as a result of the epoxy crosslinking of theinvention.

[0091] The results of Examples 7 to 10 are summarized in Table 3 below.TABLE 3 SST 10 N, SST 10 N, 70° C. BS steel Gel value [min] [min] [N/cm][%] Example (test A1) (test A2) (test B) (test C) 7 +10000 6590 3.8 67 8^(a) 345 55 4.1 39  8^(b) 1285 160 4.2 59 9 +10000 +10000 3.6 71 10 +10000 +10000 3.7 68

[0092] For Examples 7, 9 and 10, the converse route was taken. Bycopolymerization of the hydroxyethyl methacrylate (HEMA), hydroxylgroups were incorporated randomly along the polymer chain and were thencrosslinked using a difunctional epoxide with acid catalysis. Via thisroute as well, the crosslinking reaction of the invention proceeds withsignificantly greater efficiency than the electron beam curingcomparison which was conducted. Moreover, here again, differentcomonomer compositions are tolerated. Example 10 emphasizes thatcompositions with a relatively high degree of regulation can beprocessed as hotmelts and, with the crosslinking, likewise extend intothe high shear strength range.

[0093] A variant of the process which has been found particularlyefficient is that wherein the epoxy-functionalized polyacrylates werereacted with hydroxy-functionalized crosslinkers (alcohols).

1. A process for preparing crosslinked polyacrylates, in which anacrylate-based monomer mixture is copolymerized to produce a polymerand, after the polymerization, crosslinking of the polymer is broughtabout by UV radiation, characterized in that up to 10% by weight ofcopolymerizable monomers containing one or more epoxy groups and/or oneor more hydroxyl groups being incorporated into the polymers during thecopolymerization and, prior to crosslinking, at least one photocationgenerator and also one or more di- or polyfunctional epoxides and/oralcohols are added.
 2. The process of claim 1, characterized in that thecomposition of the monomer mixture is such that the resulting polymerspossess pressure sensitively adhering properties.
 3. The process of atleast one of the preceding claims, characterized in that thepolyacrylates for crosslinking are polyacrylates which have beenprocessed and worked on prior to crosslinking in the hotmelt process. 4.The process of at least one of the preceding claims, characterized inthat the composition of the monomer mixture is as follows: a) 60 to99.5% by weight of (meth)acrylic acid and/or (meth)acrylate esters ofthe following formula

 where R¹=H or CH₃ and R² is an alkyl chain having 1 to 20 carbon atoms,in particular having from 4 to 14 carbon atoms, very particularly having4 to 9 carbon atoms, b) 0.5 to 10% by weight of copolymerizable monomerswhich contain one or more epoxy groups and/or one or more hydroxylgroups, c) optionally up to a maximum of 39.5% by weight ofcopolymerizable olefinically unsaturated monomers containing functionalgroups which activate the double bond for the polymerization reaction,the components of the monomer mixture adding up to 100%, and/or if,prior to crosslinking, d) 0.01 to 25% by weight of a photocationgenerator and e) 0.1 to 5% by weight of one or more di- orpolyfunctional epoxides and/or one or more di- or polyfunctionalalcohols are added to the polymers, so that the polymers and componentsd) and e) add up to 100% by weight.
 5. The process of at least one ofthe preceding claims, characterized in that a triarylsulfoniumhexafluoro salt from group 15 of the periodic system, especiallytriarylsulfonium hexafluorophosphate and/or triarylsulfoniumhexafluoroantimonate, are used as photocation generators.
 6. The processof at least one of the preceding claims, characterized in that, in thecourse of crosslinking, structuring of the crosslinked polyacrylates isachieved by covering the polyacrylates to be crosslinked with a maskhaving regions of different UV transparency, the structuring of thepolyacrylates consisting in the presence within the polyacrylates ofregions of high crosslinking alongside regions of low crosslinkingand/or noncrosslinked regions.
 7. The use of di- or polyfunctionaloxygen compounds, particularly of di- or polyfunctional epoxides oralcohols, as crosslinking reagents for the crosslinking reaction,brought about by ultraviolet radiation in the presence of a photocationgenerator, of polyacrylates functionalized by epoxy groups and/orhydroxyl groups.